The long term goal of the proposed research is a molecular understanding of how the mutation rate of an organism is defined and how this basal mutation rate is increased in response to environmental mutagenic agents. The bacterium E. coli has been chosen for the study of these questions because it provides a relatively simple experimental system amenable to combined genetic and biochemical approaches. Specific aims of the grant are to learn: (1) enzymatic mechanisms that define the fidelity of DNA replication; (2) regulation and scope of the induced SOS response to DNA damage, which includes increased mutation rate and enhanced capacity for DNA repair. Work on replication fidelity will focus initially on a biochemical analysis of the Alpha and Epsilon subunits of polymerase III holoenzyme, which appear to be the primary determinants for the accuracy of normal replication in E. coli. The contribution and possible regulation of the 3' Yield 5' exonuclease (editing function) will be of special concern. Study of the SOS response will seek to define: (i) the molecular signal of DNA damage that initiates the SOS response; (ii) the biochemical basis for the increase in replication errors; (iii) the scope of the SOS response in terms of other types of genetic variation, such as transposition and duplication. An understanding of mutation rate and its control is likely to be pertinent for an understanding of initiating events in cancer. Many agents that are carcinogenic in animals activate the SOS response in bacteria. Cancer might often derive from the aberrant activation of an SOS-like system in somatic cells, the normal purpose of which is inducible genetic variation in germ cells.